1. Field of the Invention
The present invention generally relates to output parts of a semiconductor integrated circuit such as a power switch IC used for power-control/power-saving-control purposes, and particularly relates to a pattern shape of a set of MOSFET devices which particularly require a high-density implementation.
2. Description of the Related Art
Output parts of power switch ICs need to possess great driving power, and, thus, is comprised of a large number of MOSFET devices.
FIG. 2 is an illustrative drawing showing a configuration of output parts of a related-art power switch IC.
As shown in FIG. 2, MOSFET devices are arranged in a matrix formation at high density within a limited space. Source regions are connected via wires between different devices, and so are drain regions, thereby forming a large single transistor device.
FIG. 3 is an illustrative drawing showing an enlarged view of MOSFET devices which are arranged as the output parts of the related-art power switch IC.
In FIG. 3, source electrodes 14 and 15 cover source regions of MOSFET devices, which exist beneath the source electrodes 14 and 15. Likewise, drain electrodes 16 and 17 cover drain regions of the MOSFET devices, which underlay the drain electrodes 16 and 17. These electrodes are formed from an aluminum layer. A wire 18 diagonally connect adjacent drain electrodes 16 and 17, and is formed from the same aluminum layer. Contact holes 19 and 20 connect between the source electrodes 14 and 15 and the underlaying source regions, respectively. Contact holes 21 and 22 connect between the drain electrodes 16 and 17 and the underlaying drain regions, respectively.
The wire 18 is diagonally laid out in order to help to shorten a distance between MOSFET devices, thereby achieving a high packing density of the MOSFET devices. The extent to which the MOSFET devices are packed in a compact space will determine the overall area size of the output parts. The higher the packing density, the smaller the area size of the power switch IC is.
In a MOSFET pattern, however, there is a limit to how short a tolerable distance can be between regions. A minimum tolerable distance is governed by such factors as limits of lithography or etching processes as well as distances necessary to insure electrical insulation.
With reference to FIG. 3, a description will be given below with regard to distances between regions of MOSFET devices and associated problems.
In FIG. 3, a distance between a perimeter of the contact holes 19 and 20 and source electrodes 14 and 15, respectively, is denoted as 24. This distance 24 must be longer than such a minimum tolerable distance as a margin of error for relative positioning requires. A distance 25 is the shortest distance between the wire 18 and either one of the source electrodes 14 and 15. Since the wire 18 and the source electrodes 14 and 15 are formed in the same layer, the distance 25 must be longer than such a minimum tolerable distance as electrical insulation can be secured. A distance 26 between the source electrodes 14 and 15 and the drain electrodes 16 and 17 represents a distance between the MOSFET devices arranged in a matrix.
In the related-art configuration of the MOSFET devices as shown in FIG. 3, if the distance 26 between the MOSFET devices is shortened, the rectangular source electrodes 14 and 15 may have corners thereof approaching too close to the wire 18, so that the distance 25 may become shorter than the minimum tolerable distance. In order to keep this minimum tolerable distance, intervals at which the MOSFET devices are arranged cannot be shortened than certain limits. There is a limit, therefore, to a reduction in a device size as long as the related-art configuration of MOSFET devices is used. It should be noted that the same argument applies to a distance between the drain electrodes 16 and 17 and a diagonally installed wire (not shown) connecting source regions together.
Accordingly, there is a need for a semiconductor integrated circuit which allows MOSFET devices forming output parts thereof to be densely arranged, thereby reducing an overall size of the output parts.